岩石润湿性的核磁共振表征方法与初步实验结果

Nuclear magnetic resonance characterizes rock wettability:preliminary experimental results

随着石油勘探开发领域的不断扩大,某些复杂油气藏尤其是致密油气的岩石物理响应,受岩石润湿性变化的影响越来越明显,影响油气藏的评价以及后续勘探开发,识别和评价岩石润湿性成为重要的研究课题.核磁共振弛豫可以有效表征多孔介质孔隙及其流体分布,特别是通过T1-T2弛豫图谱能够获得流体分子与孔隙表面相互作用的信息,进而为孔隙表面润湿性质的判断提供了可能性.本文首先讨论了T1-T2弛豫谱表征孔隙介质润湿强度的理论基础,然后,采用一阶近似方法对孔隙表面流体分子的核磁共振弛豫分布进行正演模拟,建立T1/T2比值法判别孔隙介质润湿性的基本方法.最后,选取玻璃珠和天然砂岩样品进行初步实验,验证方法的合理性.结果表明,当岩石饱和双相流体时,润湿相流体T1/T2值高于非润湿相流体,通过观察不同饱和度状态T1-T2二维图谱的变化,能够判断岩石的润湿性.本文的理论分析与初步实验结果为岩石润湿性研究提供了新的手段,对核磁共振测井探测储层润湿性也具有一定的应用价值.

With expansion of oil exploration and development fields,the influence of wettability on petrophysical parameters in complicated reservoirs is getting increasingly evident,which affects reservoir evaluation and subsequent exploration and development.Thus,rock wettability identification and evaluation has become an important research topic.Nuclear Magnetic Resonance(NMR)relaxation can effectively characterize pores and fluid distribution in porous media.In particular,the interaction strength between pore surface and fluid molecules could be obtained through T1-T2 correlation spectra,which provides a possibility to judge the wettability of pore surface.In this paper,the theoretical basis that T1-T2 correlation was used to characterize the wetting features of porous media is discussed.Then,the relaxation correlation of fluid molecules on pore surface is simulated by the first order approximation method.And the basic method for distinguishing the wettability is established using T1/T2 ratio.Finally,glass beads and reservoir sandstone samples are selected for preliminary NMR experiments to verify the validity of the method.The results show that the T1/T2 ratio of the wetting-phase fluid is higher than that of the non-wetting phase fluid when rocks are saturated with two-phase fluids.Rock wettability is determined by observing the changes of T1-T2 correlation spectra in different saturation states.The theoretical analysis and preliminary experimental results in this paper provide a novel method for the study of rock wettability,and have certain application value in the detection of reservoir wettability via NMR logging.